Common Stock (Details) - USD ($)	3 Months Ended		6 Months Ended		45 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Dec. 31, 2014	Oct. 23, 2014	Dec. 05, 2013	Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items]
Common stock, outstanding	62,784,449		62,784,449		62,784,449	62,981,229
Common stock, issued	102,562	112,108	281,490	562,724
Number of shares repurchased			500,000		8,000,000
Stock repurchased (in US dollars)			$ 21,148,000	$ 18,724,000	$ 304,700,000
RepurchaseProgramSep262011
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)									$ 100,000,000
RepurchaseProgramDec052013
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)								$ 100,000,000
RepurchaseProgramOct232014
Equity, Class of Treasury Stock [Line Items]
Stock repurchase, authorized amount (in US dollars)							$ 250,000,000
Remaining authorized repurchase amount (in US dollars)	$ 145,300,000		$ 145,300,000		$ 145,300,000